Building a stable case climate

Conditioning a building is expensive, energy-hungry and often impossible in historic fabric. Conditioning a display case is cheap, passive and entirely achievable. For a collection where a small number of objects need better conditions than the room provides, the case is almost always the right unit of intervention.

The three variables

Air exchange rate. How often the case swaps its air with the room. This is the master variable and everything else follows from it. A case at one air change per day behaves entirely differently from one at ten, and the difference is not in the specification sheet, it is in the workmanship.

Buffer quantity. How much conditioned sorbent is inside, expressed per cubic metre of case volume. Silica gel and sorbents covers the specification, but the honest headline is that quantity compensates for leakage only up to a point and then stops helping.

Room conditions. The case slows the approach to the room’s average; it does not change it. A case in a gallery averaging 65 percent will end up near 65 percent eventually, however much gel is in it.

Getting the seal

Leakage in practice comes from the same handful of places: the door or access panel, the joint between the glazing and the frame, cable entries for lighting, and any deliberate ventilation left over from an earlier design philosophy.

Gaskets on the access panel do most of the work and are the easiest thing to improve on an existing case. Cable entries are the most commonly overlooked, because a hole for a lighting lead is invisible from the front and defeats an otherwise good case. Lighting mounted outside the case, shining in through the glazing, removes both the cable problem and the heat problem at once.

An important caution: sealing an existing case changes its pollutant behaviour, and improving the seal on a case built from untested materials can turn a tolerable environment into a harmful one. If you do not know what the case is made of, find out before sealing it.

Proving it works

A case whose interior is not logged is an intention rather than a control. The sequence that produces evidence:

1. Put a logger inside before doing anything, and log for a month alongside the room. This establishes the starting behaviour and the leak rate.
2. Condition the buffer to the target and install it.
3. Log for a further three months without touching anything. The curve tells you both whether the target was reached and how fast it is drifting.
4. From the drift rate, set the reconditioning interval, and write it in a maintenance schedule rather than relying on memory.

Radio-readout loggers earn their extra cost here, because reading the case without opening it avoids resetting the experiment every time you want a number.

The pollutant half of the problem

Everything that makes a case good at holding humidity makes it good at holding acetic acid. This is not a reason to avoid sealing, it is a reason to do two things at once.

Case materials that will be enclosed long-term should be Oddy tested, or chosen from the established safe list of glass, powder-coated steel, anodised aluminium, polyester, polyethylene and polypropylene. Where an existing wooden case cannot be replaced, sealing the internal surfaces and adding pollutant sorbents alongside the humidity buffer is the pragmatic compromise, with the sorbent on a replacement schedule.

Objects made of lead, shell or calcareous material are the ones to worry about first, and metals generally are covered in materials: metals.

When a case is the wrong answer

If the whole store is damp, a case will not save the collection, it will merely delay the same outcome for a few objects. If the room swings but averages something acceptable, the case is exactly right. If the room is stable at a bad value, only the room can be fixed.

The distinction is visible in a year of logged data, which is one more reason climate monitoring comes before any purchase.